It combines mv and sed to rename files based on patterns in their names. Invoke thus:

cd config/locales/en
mvsed s/yml/en.yml/g *.yml

Repeat for each language, and then just move all the yml files up to the parent directory, and you're done. You can use this for lots of renaming operations, and as you will have noticed, you don't need to be a sed guru to figure out basic patterns. This script works with bash on my macos 10.5; I imagine it will work with little alteration on the sensible non-MSDOS shell you're using.